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An improved core sampling technique for soil magnetic susceptibility determination
Geoderma 277 (2016) 35–40
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Geoderma journal homepage: www.elsevier.com/locate/geoderma
An improved core sampling technique for soil magnetic susceptibility determination Liang Liu, Keli Zhang ⁎, Zhuodong Zhang State Key Laboratory of Earth Surface Processes and Resource Ecology, School of Geography, Beijing Normal University, Xinjiekouwai Str. 19, 100875 Beijing, China
a r t i c l e
i n f o
Article history: Received 19 October 2015 Received in revised form 15 December 2015 Accepted 27 April 2016 Available online xxxx Keywords: Magnetic susceptibility measurement Technique improvement Core sampling Soil science
a b s t r a c t The efficiency of acquiring soil magnetic susceptibility (MS) data is strongly influenced by time-consuming sampling and treatment procedures of the conventional MS technique (M1). It is necessary to find an optimized technique to save time and labor in soil-related research, especially at large scales with a large number of soil samples. An improved soil sampling technique (M2) based on an improved core sampler kit and a simplified sample treatment procedure was introduced. Sample treatment of M1 is similar to other common soil- and sedimentprocessing procedures, while M2 involves relatively limited procedures without grinding and sieving. A total of 360 samples from 5 soil layers at 12 sampling sites based on M1 and M2 were collected on a gentle cultivated slope in Northeast China. The tested black soils contain few gravel (N 2 mm) and other non-soil components. Results showed that M2 saves a considerable amount of time and labor for MS determination compared with M1. Assuming one person, 85% of the time processing 10,000 soil samples using M1 can be saved by using M2. Additionally, M2 produces MS data (χlf, χfd%) with satisfactory accuracy compared with M1. Pearson correlation coefficients for χlf and χfd% between M1 and M2 are 0.973 and 0.882 (n = 180) at the 0.01 level of significance, respectively. The mean relative error in χlf and χfd% based on M2 is 8.5% and 8.2%, respectively, when compared with M1. These results indicate that M2 for MS determination is effective for soil related research at large spatial scales where large sample sets are required or representative samples are required in advance for other timeconsuming or expensive analyses. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Magnetic susceptibility (MS) is a very convenient parameter to identify the type of material and the amount of iron-bearing minerals and has been widely used in current environmental studies as a tracer (Evans and Heller, 2003). In the 1950s, Le Borgne (1955) first introduced MS into soil research and corroborated that topsoil often bears greatly enhanced magnetism compared with the bedrock on which it formed. Although many scientists paid attention to MS application technique in soil science, no remarkable findings were reported until the 1980s when instruments used in magnetic laboratories became commercially available worldwide. From then on, an increasing number of soil and environmental scientists have attempted to find better means to quantitatively link soil to environmental factors based on the MS technique, and the MS technique became popular in soil-related studies including paleoclimatic field studies (An et al., 1991; Heller and Liu, 1986; Huang et al., 2006; Maher, 1998; Torrent et al., 2010; Xiao et al., 1995), soil surveys (Blundell et al., 2009; Dearing et al., 1996; Dearing et al., 1995; Fine et al., 1989; Kämpf and Schwertmann, 1983; ⁎ Corresponding author. E-mail addresses: [email protected] (L. Liu), [email protected] (K. Zhang), [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.geoderma.2016.04.030 0016-7061/© 2016 Elsevier B.V. All rights reserved.
Marques et al., 2014; Mathé and Lévêque, 2003), soil erosion and redistribution studies (de Jong et al., 1998; Dearing et al., 1986; Jordanova et al., 2014; Liu et al. 2015; Rahimi et al., 2013; Royall, 2001) and archaeological prospecting (Chianese et al., 2004; Tite and Mullins, 1971). The wide application of the MS technique shows its reliability and feasibility in various research fields, and it is cheap and efficient in comparison to other tracing techniques such as radioactive tracers. Conventional sample pretreatment for soil MS analysis is similar to traditional soil sample preparation procedures of most soil analyses of physical and chemical properties including grinding and sieving, which are usually time-consuming and commonly more than three days, although MS determination for one sample requires less than 10 s on the instrument (Dearing, 1994). The conventional auger sampling technique for MS measurement (M1) contains three subprocesses, including field sampling, sample preparation and measurement in the laboratory. According to our experience based on thousands of conventional sample MS analyses (Liu et al. 2015), majority of the time consumed in the MS analysis procedure is the sample preparation in laboratory, which accounts for approximately three-quarters of the total time for each sample. If the quantity of samples is small, the conventional technique is acceptable, but for large amounts of samples, the analysis time will rapidly increase. Moreover, from the health and safety perspective, it is potentially harmful for operators to inhale the
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inevitable but considerable amount of aerosols (fine dusts mostly) during sample preparation in the soil sample processing laboratory. For these reasons, it is imperative to develop a new approach to MS analysis that can save time and produce satisfactory MS data in soilrelated studies at large spatial scales. The objectives of this paper were to introduce an improved soil core sampling technique based on an improved core sampler kit (M2) to measure MS and to confirm the reliability and precision of this improved technique compared with M1.
To acquire soil MS data with wide value range, soil samples at 12 sites which cover main slope positions on a typical gentle slope were collected in early October in 2014. M1 (Fig. 2) and M2 (Fig. 2) were both implemented at these sites. At each site, a soil profile of 50 cm in depth at an interval of 10 cm was sampled with three replications by both the gouge auger and the core sampler. A 3 cm soil segment was collected from the middle of each 10 cm interval. In total, 360 samples were obtained, including 180 auger soil samples and 180 paired core soil samples.
2. Materials and methods 2.3. Sample treatment and measurement 2.1. An improved core sampler kit Bartington MS2 magnetic susceptibility meter was used to determine MS, which works stably with high precision and has been widely applied (Dearing, 1994). According to our experience, the sample preparation procedures of M1 including grinding and sieving are timeconsuming for large sample set. Thus, to achieve the MS measurement without grinding and sieving, an improved core sampler and its PVC (Polyvinyl Chloride) sample box (Fig. 1) were designed to collect core samples according to the size of the sample cavity in a Bartington MS2B sensor for direct MS measurement (Liu and Zhang, 2015). During sampling, one PVC ring with the core sampler was forced into the soil to a specific depth, and it became fully filled with undisturbed soil. The core sampler was pulled out and the PVC ring with one core sample was easily withdrawn. Then the core sample in one PVC sample box was acquired for MS determination in the laboratory.
Samples obtained by M1 (Fig. 3) were disaggregated by hand or grinded lightly and were then spread on a piece of paper to be air dried for three days. After passing the samples through 2 mm mesh nylon sieves, they were sub-sampled to fully fill in the standard 10-cm3 plastic cylinder container. Core samples obtained by M2 (Fig. 3) in 9.5-cm3 cylinder PVC boxes were oven-dried at 40 °C to constant weights. MS parameters of the samples were determined using a Bartington MS meter and a dual frequency MS2B sensor with maximum measurement resolution of 0.1 × 10−5 SI units. Volume-specific magnetic susceptibilities at low frequency (0.47 kHz; κlf) and high frequency (4.7 kHz; κhf) were determined. After calculating the bulk density (ρ) of one soil sample in a fixed-volume sample box, mass-specific MS (χlf, χhf) was calculated by equation χ = κ/ρ. Then, percentage frequency-dependent MS χfd% was calculated by equation χfd % = (χlf − χhf)/χlf ×100 (Dearing, 1994).
2.2. Sampling
2.4. Data analysis
The sampling site (125°18′51″ - 125°20′39″E, 48°59′59″ 49°00′50″N) is located on a typically cultivated land in Nenjiang County, Heilongjiang province, Northeast China. This region is dominated by a semi-humid temperate climate and undulating topography with gentle and long slopes. The black soil in Northeast China has a clayloamy to loamy-clay texture with negligible gravel (N2 mm) (NSSO, 1998). Detailed description of the black soil has been shown in the previous MS study (Liu et al. 2015) in this region.
Time efficiency and data accuracy were the main concerns for comparing the two techniques. Time efficiency of the two techniques was assessed by analyzing their specific steps and in scenarios of different research scales. Precision of the M1 and M2 data sets was evaluated by coefficient of variations (CV) of the triplicate soil samples at each sampling site. The correlation of data sets between M1 and M2 was evaluated by Pearson correlation coefficient, absolute error (AE) and relative error (RE).
Fig. 1. Outline drawing of the core sampler and its PVC sample box for collecting soil sample of magnetic susceptibility.
L. Liu et al. / Geoderma 277 (2016) 35–40
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Fig. 2. Photos of the improved core and conventional auger sampling techniques for soil magnetic susceptibility measurement. Tools used in the improved core sampling technique include a hammering head and its extension rods (a), a core sampler (b′), a PVC ring and its PVC cover (d′). Tools used in the conventional auger sampling technique include a gouge auger (a & b), a nylon mesh (e) and a standard specimen box for the Bartington MS2B sensor (f).
3. Results and discussion 3.1. Comparison of time efficiency between auger and core sampling techniques The amount of time for sample preparation of M2 is obviously shortened compared to M1. According to our practice, field sampling time of
M1 and M2 was similar. However, in the laboratory, M2 does not require complex procedures, including grinding, sieving and sample box filling (Fig. 3). Because the volume of the tested sample should be known for calculation, and the tested soil must be filled into a dedicated MS box with a fixed volume according to the requirements of standard MS measurement using the MS meter (Dearing, 1994). Thus, if the soil samples by conventional auger sampling with irregular shapes are not
Fig. 3. Simplified flow chart showing the major stages of auger and core sampling techniques for soil magnetic susceptibility measurement.
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Table 1 Time summary of sample preparation based on auger (M1) and core (M2) sampling techniques for magnetic susceptibility determination at different spatial scales. Technique
M1
Process
Step 1
Time
Air dry of disturbed soil samples
Step 2
M2
Per 100 samples at slope scale
Per 1000 samples at small watershed scale
2 h (0.25 days) for dispersing wet soil samples on clear white paper; 3 days for air dry 4 h (0.5 days) 4 h (0.5 days) 1 h (0.125 days)
20 h or 2.5 days for dispersing 200 h or 25 days or 1 month for wet soil samples on clear white dispersing wet soil samples on clear papers; 6 days for air dry papers; 28 days or 1 month for air drya 40 h (5 days) 400 h (50 days or 2 months) 40 h (5 days) 400 h (50 days or 2 months) 10 h (1.25 days) 100 h (12.5 days or 0.4 months)
Hand disaggregation/lightly grind Sieve (passed through 2-mm mesh sieve) Step 3 Weight plastic sample boxes of magnetic susceptibility Step 4 Sub-sampling 1 h (0.125 days) Fill plastic sample boxes with sieved samples 1 h (0.125 days) Step 5 Weight plastic sample boxes filled with 1 h (0.125 days) sieved samples Step 6 Measure magnetic susceptibility 2 h (0.25 days) In sum About 5 days Step 1 Oven dry intact core samples at 40 °C 0.1 h for putting wet core samples in oven; 2 days for oven dry Step 2 Weight plastic sample boxes filled 1h with core samples Step 3 Measure magnetic susceptibility 2h In sum 2.5 days Time-saving efficiency compared with 58% conventional auger sample processing
Per 10,000 samples at regional scale
10 h (1.25 days) 10 h (1.25 days) 10 h (1.25 days)
100 h (12.5 days or 0.4 months) 100 h (12.5 days or 0.4 months) 100 h (12.5 days or 0.4 months)
20 h (2.5 days) About 20 days 0.2 h for putting wet core samples in oven; 2 days for oven dry 10 h (1.25 days)
200 h (25 days or 1 month) About 10 months 1 h for putting wet core samples in oven; 2 days for oven dry 100 h (12.5 days or 0.4 months)
20 h (2.5 days) About 6 days 70%
200 h (25 days or 1 month) About 1.5 months 85%
Note: given that all the experiments are carried by one person and 8 h is used as a working day. a Represents the number of ovens could meet the requirements of drying 10,000 core samples simultaneously.
Table 2 Statistics for the magnetic susceptibility data based on auger (M1) and core (M2) sampling techniques. MS indicator
Technique
N total
Minimum
Maximum
Mean
SD
CV
Skewness
Kurtosis
Normality test
χlf
M1 M2 M1 M2
180 180 180 180
9.6 9.1 2.8 3
88.3 79.1 11.1 11
26.7 25.8 7.3 7.4
17.6 16.3 2.2 2.1
0.66 0.63 0.30 0.28
1.6 1.6 −0.2 −0.2
2.3 1.9 −1.1 −1.1
NN NN N N
χfd%
Note: χlf in 10−8 m3 kg−1and χfd% in %. N total denotes the total number of samples. SD denotes standard deviation. CV denotes coefficient of variation. N indicates that the data statistically follows normal distribution and NN indicates that the data statistically does not follow normal distribution at the 0.01 level of significance.
grinded, sieved and filled into the dedicated box, the mass-specific MS data (χlf, χfd%) cannot be precisely measured. Assuming one person working on sample preparation, the total amount of sample preparation time of M1 was shortened by 58%, 70% and 85% on slope, small watershed and regional scales, respectively (Table 1). The sample processing procedure of M2 includes only three steps, which saved large amounts of time and labor. The treatment of core samples of M2 can be easily handled by one person so that one can deal with hundreds of core samples simultaneously. Low oven drying temperature (at 40 °C) avoids the conversion of iron oxide in soils, which would potentially change their original MS. M1 not only requires more time and labor but also has side-effects on the operator's health, which is often overlooked. The fine dusts from the processes of soil sample grinding, sieving and grubbing is potentially harmful to operators, such as chest tightness and coughing (Pope and Dockery, 2006).
3.2. Accuracy comparisons of the MS values from auger and core sampling techniques MS results determined by M1 and M2 have good agreement statistically (Table 2). Specifically, the mean, SD and CV values of χlf data based on M2 account for 96.6%, 92.6% and 95.5% of M1, and the mean, SD and CV values of χfd% data based on M2 account for101.4%, 95.5% and 93.3% of M1. χlf of M1 is slightly higher than M2. Because for the black soils in this study, they have typical pedogenic profiles and the highest MS values appear at top layers which contain non-soil materials, including
few gravel (N2 mm), plant residues and other soil intrusive bodies which have very low MS values. These materials were removed from the black soil by the processing procedure of M1. The remaining
Table 3 Statistics for the coefficient of variation (CV) data of magnetic susceptibility of the triplicate soil samples at each sampling point based on auger (M1) and core (M2) sampling techniques. χlf
Statistic
N total Mean Standard deviation Minimum Maximum Percentiles
10 20 25 30 40 50 60 70 75 80 90
χfd%
M1 %
M2 %
M1 %
M2 %
60 0.08 0.06 0.00 0.27 0.02 0.03 0.03 0.04 0.05 0.05 0.07 0.09 0.10 0.13 0.16
60 0.12 0.10 0.01 0.44 0.03 0.03 0.04 0.05 0.06 0.09 0.11 0.14 0.15 0.19 0.25
60 0.09 0.08 0.00 0.36 0.01 0.02 0.02 0.03 0.04 0.06 0.07 0.10 0.12 0.15 0.22
60 0.09 0.09 0.00 0.38 0.01 0.02 0.02 0.03 0.05 0.06 0.09 0.12 0.14 0.16 0.22
Note: CV denotes coefficient of variation. N total denotes the total number of samples.
L. Liu et al. / Geoderma 277 (2016) 35–40 Table 4 Statistics for error data of magnetic susceptibility of the core sampling technique (M2) compared with the auger sampling technique (M1) based on the mean values of triplicate soil samples at each sampling site. Statistic
N total Mean Standard deviation Minimum Maximum Percentiles 10 20 25 30 40 50 60 70 75 80 90
χfd%
χlf AE 10−8 m3 kg−1
RE %
AE %
RE %
60 2.0 1.9 0.1 8.5 0.2 0.5 0.6 0.8 1.2 1.5 1.9 2.7 2.8 3.7 4.5
60 8.5 7.5 0.4 37.0 0.9 1.8 3.5 4.2 5.4 6.2 8.4 10.0 11.4 13.6 21.1
60 0.5 0.4 0.0 1.6 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.2
60 8.2 8.9 0.3 35.7 0.9 1.7 2.1 2.2 3.4 4.7 6.5 9.6 12.1 14.9 18.9
Note: AE denotes absolute error. RE denotes relative error. N total denotes the total number of samples.
ferrimagnetic minerals in the soil were concentrated, resulting in the slightly higher χlf of M1. CV values of the MS data (χlf, χfd%) were used to evaluate the data precision of the triplicate samples at each sampling site (Table 3). The mean CV values of χlf data base on M1 and M2 are 0.08 and 0.12. The mean CV values of χfd% based on M1 and M2 are both 0.09. This indicates that M2 has a similar level of precision as M1. The slightly higher CV values of M2 than M1 might be caused by the non-soil materials like gravel and grass roots which stayed in the core samples. Table 4 presents the precision analysis by M2 results assuming those based on M1 as standards. The mean AE of χlf and χfd% are 2.0 × 10−8 m3 kg−1 and 0.5% respectively, and the mean RE values of χlf and χfd% are 8.5% and 8.2% respectively. This shows that most of MS data based on M2 are of high consistency with those of M1. A positive linear correlation exists in the MS data between M1 and M2 (Fig. 4). Pearson correlation coefficients for χlf and χfd% between M1 and M2 are 0.973 and 0.882 respectively. The slightly lower correlation coefficient of χfd% than χlf can probably be attributed to the multiplier effects by the mathematical computation based on χlf data. Therefore, M2 is accurate and reliable, and it can be used as an alternative to M1 for measuring MS.
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3.3. Theoretical foundation and application prospect of the core sampling technique Like common soil- and sediment-processing procedures, conventional sample preparation for MS measurement is aimed to keep soil sample homogeneous and to avoid the influence of non-soil materials, including soil moisture, plant roots, and gravel (N2 mm) (Carter and Gregorich, 2007). In practice, much time and work are required during the sample processing procedures including grinding and sieving. However, results based on M2 showed that such procedures excluding grinding and sieving resulted in only slight data deviation but saved a considerable amount of time. The magnetic materials in the tested black soils nearly hardly changed at all during the transformation from intact core soil to sieved soil. In general, soils are composed of minerals, organic matter, water, gravels and other materials. The total mass-specific MS in soils is largely determined by the ferrimagnetic minerals as their MS is 1000 times greater than that of other iron oxides (Mullins, 1977), and the massspecific MS of non-soil materials like organics, quartz and water are negligible (Dearing, 1994), and soil aggregate size should not affect soil MS. Thus, for most cropland soils with few gravel, plant roots and other non-soil materials, the procedures including grinding and sieving can be omitted when measuring MS. Dearing et al. (1996) also mentioned that the MS of milled or unmilled soils should show no significant difference based on a national data set. From a practical point of view, rapid MS determination of M2 for large sample sets (i.e. more than 1000 samples) is valuable, as time and labor constraints in conventional sample pretreatment for MS determination often restrict research plans for large amount of samples for MS analysis. However, the total mass of single core sample for MS is small and it cannot provide enough soil material for analyses of other indicators. If other time-consuming or expensive soil analyses are required, carrying out preliminary experiments using M2 would be effective, then researchers can consider whether M1 should be employed to acquire more samples for multiple-indicator analysis in the laboratory. 4. Conclusion An improved core sampling technique for measuring MS in soils based on an improved core sampler kit was introduced. By comparing the time consumed and data accuracy between conventional auger sampling technique (M1) and improved core sampling technique (M2), we conclude that the M2 for MS determination greatly improved the measurement efficiency of MS, that is, the total time of M1 was
Fig. 4. Comparison of magnetic susceptibility (χlf, χfd%) data determined by auger (M1) and core (M2) sampling techniques.
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reduced by 58% at slope scale, 70% at small watershed scale and 85% at regional scale using M2 depending on spatial scales. Additionally, with M1 as the baseline, the mean relative error of M2 was 8.5% for χlf and 8.2% for χfd%. These results obtained by M2 are in good agreement with M1 under the condition that the black soils in Northeast China contain few gravel (N2 mm) and other non-soil components which have very low MS values. In the long run, M2 is a promising technique and will be an alternative or paired with M1 for MS determination in multiple-indicator analysis including MS in soil redistribution and soil related research at large spatial scales. Acknowledgements We thank the National Natural Science Foundation of China (Grant No. 41471224) for funding this study and Jiusan soil conservation station of Beijing Normal University for the support of field and laboratory work. We also appreciate the constructive comments and suggestions provided by the anonymous reviewers and language improvement from Prof. Bofu Yu (Griffith University). References An, Z., Kukla, G.J., Porter, S.C., Xiao, J., 1991. Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years. Quat. Res. 36 (1), 29–36. Blundell, A., Dearing, J.A., Boyle, J.F., Hannam, J.A., 2009. Controlling factors for the spatial variability of soil magnetic susceptibility across England and Wales. Earth Sci. Rev. 95 (3–4), 158–188. Carter, M.R., Gregorich, E.G., 2007. Soil Sampling and Methods of Analysis. second ed. CRC Press, London. Chianese, D., D'Emilio, M., Di Salvia, S., Lapenna, V., Ragosta, M., Rizzo, E., 2004. Magnetic mapping, ground penetrating radar surveys and magnetic susceptibility measurements for the study of the archaeological site of Serra di Vaglio (southern Italy). J. Archaeol. Sci. 31 (5), 633–643. de Jong, E., Nestor, P.A., Pennock, D.J., 1998. The use of magnetic susceptibility to measure long-term soil redistribution. Catena 32, 23–35. Dearing, J.A., 1994. Environmental Magnetic Susceptibility, Using the Bartington MS2 System. Chi Publishers, Kenilworth. Dearing, J.A., Morton, R.I., Price, T.W., Foster, I.D.L., 1986. Tracing movements of topsoil by magnetic measurements - 2 case studies. Phys. Earth Planet. Inter. 42 (1-2), 93–104. Dearing, J.A., Lees, J.A., White, C., 1995. Mineral magnetic properties of acid gleyed soils under oak and Corsican Pine. Geoderma 68 (4), 309–319. Dearing, J.A., Hay, K.L., Baban, S.M.J., Huddleston, A.S., Wellington, E.M.H., Loveland, P.J., 1996. Magnetic susceptibility of soil: an evaluation of conflicting theories using a national data set. Geophys. J. Int. 127 (3), 728–734.
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Geoderma journal homepage: www.elsevier.com/locate/geoderma
An improved core sampling technique for soil magnetic susceptibility determination Liang Liu, Keli Zhang ⁎, Zhuodong Zhang State Key Laboratory of Earth Surface Processes and Resource Ecology, School of Geography, Beijing Normal University, Xinjiekouwai Str. 19, 100875 Beijing, China
a r t i c l e
i n f o
Article history: Received 19 October 2015 Received in revised form 15 December 2015 Accepted 27 April 2016 Available online xxxx Keywords: Magnetic susceptibility measurement Technique improvement Core sampling Soil science
a b s t r a c t The efficiency of acquiring soil magnetic susceptibility (MS) data is strongly influenced by time-consuming sampling and treatment procedures of the conventional MS technique (M1). It is necessary to find an optimized technique to save time and labor in soil-related research, especially at large scales with a large number of soil samples. An improved soil sampling technique (M2) based on an improved core sampler kit and a simplified sample treatment procedure was introduced. Sample treatment of M1 is similar to other common soil- and sedimentprocessing procedures, while M2 involves relatively limited procedures without grinding and sieving. A total of 360 samples from 5 soil layers at 12 sampling sites based on M1 and M2 were collected on a gentle cultivated slope in Northeast China. The tested black soils contain few gravel (N 2 mm) and other non-soil components. Results showed that M2 saves a considerable amount of time and labor for MS determination compared with M1. Assuming one person, 85% of the time processing 10,000 soil samples using M1 can be saved by using M2. Additionally, M2 produces MS data (χlf, χfd%) with satisfactory accuracy compared with M1. Pearson correlation coefficients for χlf and χfd% between M1 and M2 are 0.973 and 0.882 (n = 180) at the 0.01 level of significance, respectively. The mean relative error in χlf and χfd% based on M2 is 8.5% and 8.2%, respectively, when compared with M1. These results indicate that M2 for MS determination is effective for soil related research at large spatial scales where large sample sets are required or representative samples are required in advance for other timeconsuming or expensive analyses. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Magnetic susceptibility (MS) is a very convenient parameter to identify the type of material and the amount of iron-bearing minerals and has been widely used in current environmental studies as a tracer (Evans and Heller, 2003). In the 1950s, Le Borgne (1955) first introduced MS into soil research and corroborated that topsoil often bears greatly enhanced magnetism compared with the bedrock on which it formed. Although many scientists paid attention to MS application technique in soil science, no remarkable findings were reported until the 1980s when instruments used in magnetic laboratories became commercially available worldwide. From then on, an increasing number of soil and environmental scientists have attempted to find better means to quantitatively link soil to environmental factors based on the MS technique, and the MS technique became popular in soil-related studies including paleoclimatic field studies (An et al., 1991; Heller and Liu, 1986; Huang et al., 2006; Maher, 1998; Torrent et al., 2010; Xiao et al., 1995), soil surveys (Blundell et al., 2009; Dearing et al., 1996; Dearing et al., 1995; Fine et al., 1989; Kämpf and Schwertmann, 1983; ⁎ Corresponding author. E-mail addresses: [email protected] (L. Liu), [email protected] (K. Zhang), [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.geoderma.2016.04.030 0016-7061/© 2016 Elsevier B.V. All rights reserved.
Marques et al., 2014; Mathé and Lévêque, 2003), soil erosion and redistribution studies (de Jong et al., 1998; Dearing et al., 1986; Jordanova et al., 2014; Liu et al. 2015; Rahimi et al., 2013; Royall, 2001) and archaeological prospecting (Chianese et al., 2004; Tite and Mullins, 1971). The wide application of the MS technique shows its reliability and feasibility in various research fields, and it is cheap and efficient in comparison to other tracing techniques such as radioactive tracers. Conventional sample pretreatment for soil MS analysis is similar to traditional soil sample preparation procedures of most soil analyses of physical and chemical properties including grinding and sieving, which are usually time-consuming and commonly more than three days, although MS determination for one sample requires less than 10 s on the instrument (Dearing, 1994). The conventional auger sampling technique for MS measurement (M1) contains three subprocesses, including field sampling, sample preparation and measurement in the laboratory. According to our experience based on thousands of conventional sample MS analyses (Liu et al. 2015), majority of the time consumed in the MS analysis procedure is the sample preparation in laboratory, which accounts for approximately three-quarters of the total time for each sample. If the quantity of samples is small, the conventional technique is acceptable, but for large amounts of samples, the analysis time will rapidly increase. Moreover, from the health and safety perspective, it is potentially harmful for operators to inhale the
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inevitable but considerable amount of aerosols (fine dusts mostly) during sample preparation in the soil sample processing laboratory. For these reasons, it is imperative to develop a new approach to MS analysis that can save time and produce satisfactory MS data in soilrelated studies at large spatial scales. The objectives of this paper were to introduce an improved soil core sampling technique based on an improved core sampler kit (M2) to measure MS and to confirm the reliability and precision of this improved technique compared with M1.
To acquire soil MS data with wide value range, soil samples at 12 sites which cover main slope positions on a typical gentle slope were collected in early October in 2014. M1 (Fig. 2) and M2 (Fig. 2) were both implemented at these sites. At each site, a soil profile of 50 cm in depth at an interval of 10 cm was sampled with three replications by both the gouge auger and the core sampler. A 3 cm soil segment was collected from the middle of each 10 cm interval. In total, 360 samples were obtained, including 180 auger soil samples and 180 paired core soil samples.
2. Materials and methods 2.3. Sample treatment and measurement 2.1. An improved core sampler kit Bartington MS2 magnetic susceptibility meter was used to determine MS, which works stably with high precision and has been widely applied (Dearing, 1994). According to our experience, the sample preparation procedures of M1 including grinding and sieving are timeconsuming for large sample set. Thus, to achieve the MS measurement without grinding and sieving, an improved core sampler and its PVC (Polyvinyl Chloride) sample box (Fig. 1) were designed to collect core samples according to the size of the sample cavity in a Bartington MS2B sensor for direct MS measurement (Liu and Zhang, 2015). During sampling, one PVC ring with the core sampler was forced into the soil to a specific depth, and it became fully filled with undisturbed soil. The core sampler was pulled out and the PVC ring with one core sample was easily withdrawn. Then the core sample in one PVC sample box was acquired for MS determination in the laboratory.
Samples obtained by M1 (Fig. 3) were disaggregated by hand or grinded lightly and were then spread on a piece of paper to be air dried for three days. After passing the samples through 2 mm mesh nylon sieves, they were sub-sampled to fully fill in the standard 10-cm3 plastic cylinder container. Core samples obtained by M2 (Fig. 3) in 9.5-cm3 cylinder PVC boxes were oven-dried at 40 °C to constant weights. MS parameters of the samples were determined using a Bartington MS meter and a dual frequency MS2B sensor with maximum measurement resolution of 0.1 × 10−5 SI units. Volume-specific magnetic susceptibilities at low frequency (0.47 kHz; κlf) and high frequency (4.7 kHz; κhf) were determined. After calculating the bulk density (ρ) of one soil sample in a fixed-volume sample box, mass-specific MS (χlf, χhf) was calculated by equation χ = κ/ρ. Then, percentage frequency-dependent MS χfd% was calculated by equation χfd % = (χlf − χhf)/χlf ×100 (Dearing, 1994).
2.2. Sampling
2.4. Data analysis
The sampling site (125°18′51″ - 125°20′39″E, 48°59′59″ 49°00′50″N) is located on a typically cultivated land in Nenjiang County, Heilongjiang province, Northeast China. This region is dominated by a semi-humid temperate climate and undulating topography with gentle and long slopes. The black soil in Northeast China has a clayloamy to loamy-clay texture with negligible gravel (N2 mm) (NSSO, 1998). Detailed description of the black soil has been shown in the previous MS study (Liu et al. 2015) in this region.
Time efficiency and data accuracy were the main concerns for comparing the two techniques. Time efficiency of the two techniques was assessed by analyzing their specific steps and in scenarios of different research scales. Precision of the M1 and M2 data sets was evaluated by coefficient of variations (CV) of the triplicate soil samples at each sampling site. The correlation of data sets between M1 and M2 was evaluated by Pearson correlation coefficient, absolute error (AE) and relative error (RE).
Fig. 1. Outline drawing of the core sampler and its PVC sample box for collecting soil sample of magnetic susceptibility.
L. Liu et al. / Geoderma 277 (2016) 35–40
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Fig. 2. Photos of the improved core and conventional auger sampling techniques for soil magnetic susceptibility measurement. Tools used in the improved core sampling technique include a hammering head and its extension rods (a), a core sampler (b′), a PVC ring and its PVC cover (d′). Tools used in the conventional auger sampling technique include a gouge auger (a & b), a nylon mesh (e) and a standard specimen box for the Bartington MS2B sensor (f).
3. Results and discussion 3.1. Comparison of time efficiency between auger and core sampling techniques The amount of time for sample preparation of M2 is obviously shortened compared to M1. According to our practice, field sampling time of
M1 and M2 was similar. However, in the laboratory, M2 does not require complex procedures, including grinding, sieving and sample box filling (Fig. 3). Because the volume of the tested sample should be known for calculation, and the tested soil must be filled into a dedicated MS box with a fixed volume according to the requirements of standard MS measurement using the MS meter (Dearing, 1994). Thus, if the soil samples by conventional auger sampling with irregular shapes are not
Fig. 3. Simplified flow chart showing the major stages of auger and core sampling techniques for soil magnetic susceptibility measurement.
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Table 1 Time summary of sample preparation based on auger (M1) and core (M2) sampling techniques for magnetic susceptibility determination at different spatial scales. Technique
M1
Process
Step 1
Time
Air dry of disturbed soil samples
Step 2
M2
Per 100 samples at slope scale
Per 1000 samples at small watershed scale
2 h (0.25 days) for dispersing wet soil samples on clear white paper; 3 days for air dry 4 h (0.5 days) 4 h (0.5 days) 1 h (0.125 days)
20 h or 2.5 days for dispersing 200 h or 25 days or 1 month for wet soil samples on clear white dispersing wet soil samples on clear papers; 6 days for air dry papers; 28 days or 1 month for air drya 40 h (5 days) 400 h (50 days or 2 months) 40 h (5 days) 400 h (50 days or 2 months) 10 h (1.25 days) 100 h (12.5 days or 0.4 months)
Hand disaggregation/lightly grind Sieve (passed through 2-mm mesh sieve) Step 3 Weight plastic sample boxes of magnetic susceptibility Step 4 Sub-sampling 1 h (0.125 days) Fill plastic sample boxes with sieved samples 1 h (0.125 days) Step 5 Weight plastic sample boxes filled with 1 h (0.125 days) sieved samples Step 6 Measure magnetic susceptibility 2 h (0.25 days) In sum About 5 days Step 1 Oven dry intact core samples at 40 °C 0.1 h for putting wet core samples in oven; 2 days for oven dry Step 2 Weight plastic sample boxes filled 1h with core samples Step 3 Measure magnetic susceptibility 2h In sum 2.5 days Time-saving efficiency compared with 58% conventional auger sample processing
Per 10,000 samples at regional scale
10 h (1.25 days) 10 h (1.25 days) 10 h (1.25 days)
100 h (12.5 days or 0.4 months) 100 h (12.5 days or 0.4 months) 100 h (12.5 days or 0.4 months)
20 h (2.5 days) About 20 days 0.2 h for putting wet core samples in oven; 2 days for oven dry 10 h (1.25 days)
200 h (25 days or 1 month) About 10 months 1 h for putting wet core samples in oven; 2 days for oven dry 100 h (12.5 days or 0.4 months)
20 h (2.5 days) About 6 days 70%
200 h (25 days or 1 month) About 1.5 months 85%
Note: given that all the experiments are carried by one person and 8 h is used as a working day. a Represents the number of ovens could meet the requirements of drying 10,000 core samples simultaneously.
Table 2 Statistics for the magnetic susceptibility data based on auger (M1) and core (M2) sampling techniques. MS indicator
Technique
N total
Minimum
Maximum
Mean
SD
CV
Skewness
Kurtosis
Normality test
χlf
M1 M2 M1 M2
180 180 180 180
9.6 9.1 2.8 3
88.3 79.1 11.1 11
26.7 25.8 7.3 7.4
17.6 16.3 2.2 2.1
0.66 0.63 0.30 0.28
1.6 1.6 −0.2 −0.2
2.3 1.9 −1.1 −1.1
NN NN N N
χfd%
Note: χlf in 10−8 m3 kg−1and χfd% in %. N total denotes the total number of samples. SD denotes standard deviation. CV denotes coefficient of variation. N indicates that the data statistically follows normal distribution and NN indicates that the data statistically does not follow normal distribution at the 0.01 level of significance.
grinded, sieved and filled into the dedicated box, the mass-specific MS data (χlf, χfd%) cannot be precisely measured. Assuming one person working on sample preparation, the total amount of sample preparation time of M1 was shortened by 58%, 70% and 85% on slope, small watershed and regional scales, respectively (Table 1). The sample processing procedure of M2 includes only three steps, which saved large amounts of time and labor. The treatment of core samples of M2 can be easily handled by one person so that one can deal with hundreds of core samples simultaneously. Low oven drying temperature (at 40 °C) avoids the conversion of iron oxide in soils, which would potentially change their original MS. M1 not only requires more time and labor but also has side-effects on the operator's health, which is often overlooked. The fine dusts from the processes of soil sample grinding, sieving and grubbing is potentially harmful to operators, such as chest tightness and coughing (Pope and Dockery, 2006).
3.2. Accuracy comparisons of the MS values from auger and core sampling techniques MS results determined by M1 and M2 have good agreement statistically (Table 2). Specifically, the mean, SD and CV values of χlf data based on M2 account for 96.6%, 92.6% and 95.5% of M1, and the mean, SD and CV values of χfd% data based on M2 account for101.4%, 95.5% and 93.3% of M1. χlf of M1 is slightly higher than M2. Because for the black soils in this study, they have typical pedogenic profiles and the highest MS values appear at top layers which contain non-soil materials, including
few gravel (N2 mm), plant residues and other soil intrusive bodies which have very low MS values. These materials were removed from the black soil by the processing procedure of M1. The remaining
Table 3 Statistics for the coefficient of variation (CV) data of magnetic susceptibility of the triplicate soil samples at each sampling point based on auger (M1) and core (M2) sampling techniques. χlf
Statistic
N total Mean Standard deviation Minimum Maximum Percentiles
10 20 25 30 40 50 60 70 75 80 90
χfd%
M1 %
M2 %
M1 %
M2 %
60 0.08 0.06 0.00 0.27 0.02 0.03 0.03 0.04 0.05 0.05 0.07 0.09 0.10 0.13 0.16
60 0.12 0.10 0.01 0.44 0.03 0.03 0.04 0.05 0.06 0.09 0.11 0.14 0.15 0.19 0.25
60 0.09 0.08 0.00 0.36 0.01 0.02 0.02 0.03 0.04 0.06 0.07 0.10 0.12 0.15 0.22
60 0.09 0.09 0.00 0.38 0.01 0.02 0.02 0.03 0.05 0.06 0.09 0.12 0.14 0.16 0.22
Note: CV denotes coefficient of variation. N total denotes the total number of samples.
L. Liu et al. / Geoderma 277 (2016) 35–40 Table 4 Statistics for error data of magnetic susceptibility of the core sampling technique (M2) compared with the auger sampling technique (M1) based on the mean values of triplicate soil samples at each sampling site. Statistic
N total Mean Standard deviation Minimum Maximum Percentiles 10 20 25 30 40 50 60 70 75 80 90
χfd%
χlf AE 10−8 m3 kg−1
RE %
AE %
RE %
60 2.0 1.9 0.1 8.5 0.2 0.5 0.6 0.8 1.2 1.5 1.9 2.7 2.8 3.7 4.5
60 8.5 7.5 0.4 37.0 0.9 1.8 3.5 4.2 5.4 6.2 8.4 10.0 11.4 13.6 21.1
60 0.5 0.4 0.0 1.6 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.2
60 8.2 8.9 0.3 35.7 0.9 1.7 2.1 2.2 3.4 4.7 6.5 9.6 12.1 14.9 18.9
Note: AE denotes absolute error. RE denotes relative error. N total denotes the total number of samples.
ferrimagnetic minerals in the soil were concentrated, resulting in the slightly higher χlf of M1. CV values of the MS data (χlf, χfd%) were used to evaluate the data precision of the triplicate samples at each sampling site (Table 3). The mean CV values of χlf data base on M1 and M2 are 0.08 and 0.12. The mean CV values of χfd% based on M1 and M2 are both 0.09. This indicates that M2 has a similar level of precision as M1. The slightly higher CV values of M2 than M1 might be caused by the non-soil materials like gravel and grass roots which stayed in the core samples. Table 4 presents the precision analysis by M2 results assuming those based on M1 as standards. The mean AE of χlf and χfd% are 2.0 × 10−8 m3 kg−1 and 0.5% respectively, and the mean RE values of χlf and χfd% are 8.5% and 8.2% respectively. This shows that most of MS data based on M2 are of high consistency with those of M1. A positive linear correlation exists in the MS data between M1 and M2 (Fig. 4). Pearson correlation coefficients for χlf and χfd% between M1 and M2 are 0.973 and 0.882 respectively. The slightly lower correlation coefficient of χfd% than χlf can probably be attributed to the multiplier effects by the mathematical computation based on χlf data. Therefore, M2 is accurate and reliable, and it can be used as an alternative to M1 for measuring MS.
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3.3. Theoretical foundation and application prospect of the core sampling technique Like common soil- and sediment-processing procedures, conventional sample preparation for MS measurement is aimed to keep soil sample homogeneous and to avoid the influence of non-soil materials, including soil moisture, plant roots, and gravel (N2 mm) (Carter and Gregorich, 2007). In practice, much time and work are required during the sample processing procedures including grinding and sieving. However, results based on M2 showed that such procedures excluding grinding and sieving resulted in only slight data deviation but saved a considerable amount of time. The magnetic materials in the tested black soils nearly hardly changed at all during the transformation from intact core soil to sieved soil. In general, soils are composed of minerals, organic matter, water, gravels and other materials. The total mass-specific MS in soils is largely determined by the ferrimagnetic minerals as their MS is 1000 times greater than that of other iron oxides (Mullins, 1977), and the massspecific MS of non-soil materials like organics, quartz and water are negligible (Dearing, 1994), and soil aggregate size should not affect soil MS. Thus, for most cropland soils with few gravel, plant roots and other non-soil materials, the procedures including grinding and sieving can be omitted when measuring MS. Dearing et al. (1996) also mentioned that the MS of milled or unmilled soils should show no significant difference based on a national data set. From a practical point of view, rapid MS determination of M2 for large sample sets (i.e. more than 1000 samples) is valuable, as time and labor constraints in conventional sample pretreatment for MS determination often restrict research plans for large amount of samples for MS analysis. However, the total mass of single core sample for MS is small and it cannot provide enough soil material for analyses of other indicators. If other time-consuming or expensive soil analyses are required, carrying out preliminary experiments using M2 would be effective, then researchers can consider whether M1 should be employed to acquire more samples for multiple-indicator analysis in the laboratory. 4. Conclusion An improved core sampling technique for measuring MS in soils based on an improved core sampler kit was introduced. By comparing the time consumed and data accuracy between conventional auger sampling technique (M1) and improved core sampling technique (M2), we conclude that the M2 for MS determination greatly improved the measurement efficiency of MS, that is, the total time of M1 was
Fig. 4. Comparison of magnetic susceptibility (χlf, χfd%) data determined by auger (M1) and core (M2) sampling techniques.
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reduced by 58% at slope scale, 70% at small watershed scale and 85% at regional scale using M2 depending on spatial scales. Additionally, with M1 as the baseline, the mean relative error of M2 was 8.5% for χlf and 8.2% for χfd%. These results obtained by M2 are in good agreement with M1 under the condition that the black soils in Northeast China contain few gravel (N2 mm) and other non-soil components which have very low MS values. In the long run, M2 is a promising technique and will be an alternative or paired with M1 for MS determination in multiple-indicator analysis including MS in soil redistribution and soil related research at large spatial scales. Acknowledgements We thank the National Natural Science Foundation of China (Grant No. 41471224) for funding this study and Jiusan soil conservation station of Beijing Normal University for the support of field and laboratory work. We also appreciate the constructive comments and suggestions provided by the anonymous reviewers and language improvement from Prof. Bofu Yu (Griffith University). References An, Z., Kukla, G.J., Porter, S.C., Xiao, J., 1991. Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years. Quat. Res. 36 (1), 29–36. Blundell, A., Dearing, J.A., Boyle, J.F., Hannam, J.A., 2009. Controlling factors for the spatial variability of soil magnetic susceptibility across England and Wales. Earth Sci. Rev. 95 (3–4), 158–188. Carter, M.R., Gregorich, E.G., 2007. Soil Sampling and Methods of Analysis. second ed. CRC Press, London. Chianese, D., D'Emilio, M., Di Salvia, S., Lapenna, V., Ragosta, M., Rizzo, E., 2004. Magnetic mapping, ground penetrating radar surveys and magnetic susceptibility measurements for the study of the archaeological site of Serra di Vaglio (southern Italy). J. Archaeol. Sci. 31 (5), 633–643. de Jong, E., Nestor, P.A., Pennock, D.J., 1998. The use of magnetic susceptibility to measure long-term soil redistribution. Catena 32, 23–35. Dearing, J.A., 1994. Environmental Magnetic Susceptibility, Using the Bartington MS2 System. Chi Publishers, Kenilworth. Dearing, J.A., Morton, R.I., Price, T.W., Foster, I.D.L., 1986. Tracing movements of topsoil by magnetic measurements - 2 case studies. Phys. Earth Planet. Inter. 42 (1-2), 93–104. Dearing, J.A., Lees, J.A., White, C., 1995. Mineral magnetic properties of acid gleyed soils under oak and Corsican Pine. Geoderma 68 (4), 309–319. Dearing, J.A., Hay, K.L., Baban, S.M.J., Huddleston, A.S., Wellington, E.M.H., Loveland, P.J., 1996. Magnetic susceptibility of soil: an evaluation of conflicting theories using a national data set. Geophys. J. Int. 127 (3), 728–734.
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